The river mole

• source of the river mole


  Rusper
• mouth of the river mole


  molesey
• distance of river mole

  80km
• direction of river flow

  north
• cumecs at crawley

  2
• cumecs at Molesey

  7
• geology of the north downs

  chalk - permeable rock
• geology of Gatwick-dorking

  greensand (permeable) = less risk of flooding as water can flow to groundwater flow. This means after a period of rainfall, water can percolate and infiltrate into the ground, lowering the risk of flooding
• geology of dorking-molesey


  clay (impermeable) = heightened risk of flooding.
• river regime at Crawley

  subdued
• river regime at molesey

  flashy regime - due to urban areas, impermeable rock and abstraction
• when was the biggest river mole flood

  1968 - river up to 200 cumecs
• average rainfall in the UK

  1000mm
• average rainfall of river mole catchment

  769mm
• what type of drainage basin

  double tear drop - in the mole gap around Leatherhead, rainfall is often channeled through concrete channels. This increases runoff occurring here, increasing the discharge on the flood hydrograph
• land use

  1/2 agricultural, 1/4 woodland, 1/4 urban
• abstraction from the mole

  • groundwater abstraction accounts for 72% of all licensed abstraction in the mole catchment. the majority of these abstractions are from the chalk aquifer
  • the needs of public water supply accounts for 77% of licensed water abstracted in the mole catchment, almost almost all abstraction for water supply is from chalk
  • golf course irrigation also accounts for significant volume of the water abstracted
  • abstractions are also licensed for a variety of industrial uses e.g. manufacturing processes, mineral extraction
• floods of the river mole

  1968, 2000
• 2014: very high rainfall

  • transport:
  • Gatwick airport power failure, delays with luggage handling and flights
  • rail connections disrupted
  • Landslide caused embankments to collapse of the dorking to Horsham railway line: limited service and took a month to repair
  • property:
  • hotels and commercial properties in dorking and horley flooded - they were closed - cost to business
  • residential properties flooded
  • damage to planes in redhill aerodrome from winds and floods (71mph winds on north downs
  • power cuts across country - no power for 3 days
• flood management of the river mole

  • 1970s flood alleviation scheme
  • downstream in lower mole - hard engineering (weirs, bank defences at molesey) built in 1968, updated since
  • gatwick is investing £8 million in Gatwick stream flood attenuation scheme, working with the environmetal agency
  • upper mole flood alleviation scheme, £15 million environment Agency project designed to reduce flooding along the river mole, particularly in urban areas: Crawley, Horley and Gatwick airport
  • upgrading of the flood retention resoviour at Clay Lake
  • higher dam wall at Tilgate lake
• contributory factors for flooding of the river mole

  • blocked drain:
  • anecdotal evidence suggests that blocked drains caused much of the flooding in small field, near Horley during the 2013 december/january storms
  • changing land use:
  • golf courses, industrial parks, urban growth, gatwick expansion
  • urban growth:
  • increased surface run off as a rise in impermeable surfaces increase river discharge more rapidly.
  • 23% of the river mole drainage basin is urbanised, which against 1981-2010 averages, is quite low but quickly expanding
• 60% of the river mole has low permeability


  This leads to increased flood risk as there is no where for the water to infiltrate. This leads to a steeper rising limb after a period of rainfall and there is likely to be a short lag time as the discharge rapidly increases after a period of rainfall
• what is the elevation of leith hill

  265m - this will impact the flood hydrograph as there will be increased rapid surface run of and reduced infiltration and the water doesnt have time to percolate into the ground
• between ranmore, boxhill, Reigate hill the elevation is over 100m


  this increases surface run off, reduces infiltration, reduces percolation. steep rising limb and high discharge
• Urban growth impacts the hydrograph…

  areas like Horsham, horley, and the expansion of Gatwick airport terminals have increased in growth in recent years. Urban hydrograph have a flashy regime as there is high peak discharge with a short lag time. 23% of the river mole drainage basin is urban - these areas are at higher flood risk. This is because water cant infiltrate into the ground
• for urban growth to happen, areas need to be deforested


  this impacts upon the flood hydrograph as there is less vegetation to be used as interception storage and for water abstraction. this means there is increased levels of discharge in urban areas
• How does land use affect the flood hydrograph

  causes rapid runoff, decreases the lag time of rainfall into nearby streams.